1. Field of the Invention
The present invention relates to an image processing method and an image processing apparatus, and particularly to a process of determining a conversion relationship relating to data generation for a plurality kinds of color material such as ink used in an image output apparatus such as an ink jet printer, the process of determining being executed by taking account of use amounts of the plurality of kinds of color material.
2. Description of the Related Art
In general, when an image output apparatus such as a printer is used to print and output a color image, four color materials, for Y (Yellow), M (Magenta), C (Cyan), and K (black), are used. For example, an ink jet printer uses Y, M, C, and K inks, while a copy machine or a printer based on electrophotography uses Y, M, C, and K toners.
A color space realized by these color materials, used in the image output apparatus, is normally different from a color space for original image data. Thus, in the field of image processing in which color material data is generated on the basis of image data, various image processing methods have been proposed to reproduce faithfully colors or gradations shown by the original image data or to reproduce desired colors and the like.
Among these image processing methods, for example, Japanese Patent Application Laid-Open No. 2-136848 (1990) describes an image processing method that can eliminate the color reproducibility problem associated with a 100% UCR (Under-Color Removal) process executed as a part of the color material data generating process. In this method, of various combinations of Y, M, C, and K which can express a color shown by the original image data, those in which K has the highest density are defined to be the combination of Y, M, C, and K. According to this method, in contrast to the 100% UCR, where whichever one of Y, M, and C has the lowest density is replaced with K and consequently itself has a value of 0, combinations of Y, M, C, and K which do not include a value of 0 are permitted to be present. Consequently, an image with no decrease in a saturation or density can be outputted. Furthermore, compared to the image processing described in the above publication, Japanese Patent Application Laid-Open No. 6-242523 (1994) describes an image processing method of setting a value for K taking into account the reproduction ranges of the color materials Y, M, C, and K, a pseudo-contour of an output image, interpolation accuracy, and the like to obtain a further improvement in reproducibility.
However, even if image processing is executed so as to improve the reproducibility of the image output apparatus such as a printer as described above, actual printing in the printer or the like becomes improper owing to the relationship between the color materials and printing sheets. As a result, images cannot be reproduced as desired.
For example, in a process of paying attention only to the amount of K to set the amounts of the color materials Y, M, C, and K of a combination as described in the above publication, the amounts of the three color materials Y, M, and C are basically not associated with a predetermined target color as a condition that determines this combination. As a result, for example, the amounts of the three color materials Y, M, and C may vary rapidly relative to the target color, resulting in false contours.
Further, if the use amounts of the color materials cannot be ensured to vary smoothly, then for some combinations of plurality kinds of color material, the total use amount of color materials may increase greatly. In this case, for example, an ink jet printer cannot sufficiently absorb ink to a printing sheet depending on the ink absorption characteristics of the printing sheet. Thus, ink may overflow or bleed to hinder the density or the like from being accurately realized. Further, an electro-photographic printer using toner possibly cannot sufficiently fix the toner to the printing sheet, depending on the toner adhesion characteristics of the sheet. Again, the density cannot be accurately realized. For example, if the maximum amount of one color material is assumed to be 100% and a relatively dark color is printed which has signal values indicating that the amount of each color material has the maximum value, the total amount of color materials applied to the printing sheet is 400%. Where, as in this case, each of the four color materials is used at an amount close to its maximum value, then with the ink jet printer or the like, the printing sheet cannot absorb four types of inks applied so as to be superimposed on one another. Further, with the electro-photographic printer using toner, four color toners cannot be appropriately fixed to the printing sheet. As a result, the colors cannot be accurately reproduced.